ISJSON function

Structured Query Language (SQL) Server provides a powerful suite of functions to handle various data types, including JSON (JavaScript Object Notation). Among these is the ISJSON function, which is designed to validate whether a given expression contains valid JSON data. This blog will explore what ISJSON is, how it works, its practical applications, and tips for leveraging it effectively in your SQL Server projects.

What is ISJSON?

Introduced in SQL Server 2016, the ISJSON function is a logical function that tests whether a given expression is a valid JSON string. JSON has become a popular format for data interchange due to its simplicity, readability, and compatibility with modern web applications. Ensuring that stored or received data conforms to valid JSON syntax is critical for effective processing and integration, which is where ISJSON proves invaluable.

The function returns a 1 if the input expression is valid JSON, and 0 otherwise. If the input is NULL, it also returns 0.

Syntax:

ISJSON(expression)

expression: The input string to evaluate.

How ISJSON Works

ISJSON evaluates whether the given expression meets JSON syntax rules, such as proper usage of brackets ({} for objects, [] for arrays), key-value pair formatting, and proper quoting for strings. Below are examples of what ISJSON considers valid or invalid JSON.

Valid JSON Examples

A JSON object: {“name”: “John”, “age”: 30}
A JSON array: [“apple”, “banana”, “cherry”]
Nested JSON: {“person”: {“name”: “Alice”, “skills”: [“SQL”, “Python”]}}

Invalid JSON Examples

Missing quotes: {name: John, age: 30}
Malformed brackets: {“name”: “John”, “age”: 30
Trailing commas: {“name”: “John”, “age”: 30,}

Practical Applications of ISJSON

1. Validating Input Data

When receiving JSON input from APIs or front-end applications, ISJSON can help validate the data before processing. For example:

DECLARE @jsonInput NVARCHAR(MAX) = '{"name": "Jane", "age": 25}';

IF ISJSON(@jsonInput) = 1  
    PRINT 'Valid JSON';  
ELSE  
    PRINT 'Invalid JSON';

This ensures that only well-formed JSON data proceeds to subsequent operations.

2. Enforcing Data Integrity in Tables

For tables storing JSON data, ISJSON can be used in constraints to maintain data integrity.

CREATE TABLE EmployeeData (
    Id INT PRIMARY KEY,
    EmployeeInfo NVARCHAR(MAX),
    CONSTRAINT chk_EmployeeInfo_ValidJSON CHECK (ISJSON(EmployeeInfo) = 1)
);

Here, the chk_EmployeeInfo_ValidJSON constraint ensures that any data inserted or updated in the EmployeeInfo column is valid JSON.

3. Filtering Rows Containing JSON Data

ISJSON can filter rows with valid JSON data from a column:

SELECT Id, EmployeeInfo  
FROM EmployeeData  
WHERE ISJSON(EmployeeInfo) = 1;

This query retrieves rows where the EmployeeInfo column contains valid JSON, avoiding potential errors during JSON-specific operations like parsing.

Combining ISJSON with Other JSON Functions

SQL Server provides additional JSON functions, such as JSON_VALUE (extracts a value), JSON_QUERY (retrieves JSON objects or arrays), and OPENJSON (transforms JSON data into relational rows and columns). Using ISJSON alongside these functions ensures robust and error-free JSON handling.

Example:

DECLARE @json NVARCHAR(MAX) = '{"name": "Michael", "department": "HR"}';

IF ISJSON(@json) = 1  
BEGIN  
    SELECT JSON_VALUE(@json, '$.name') AS Name,  
           JSON_VALUE(@json, '$.department') AS Department;
END  
ELSE  
    PRINT 'Invalid JSON';

This script ensures data extraction only occurs if the input is valid JSON.

Tips for Effective Use

Combine with TRY/CATCH: Use error handling to manage scenarios where invalid JSON could interrupt operations.

Index JSON Data: SQL Server supports indexing JSON columns using computed columns. Validate the data with ISJSON before creating an index to prevent errors.

Preprocess Input Data: Clean and standardize incoming JSON strings before storing them in your database to reduce validation issues.

Conclusion

The ISJSON function is a simple yet powerful tool for validating JSON data in SQL Server. Its ability to identify valid JSON ensures data integrity, facilitates error-free processing, and enhances compatibility with JSON-related features in SQL Server. Whether you are working with APIs, storing structured data, or analyzing complex datasets, ISJSON provides a reliable way to maintain clean and accurate data workflows.

By incorporating ISJSON into your SQL Server toolkit, you can confidently handle JSON data and streamline your database operations.